How to keyframe child of constraint in and out
Whilst your project has the ball constrained as a Child of' the parent character (or foot) I've instead placed physics properties on the ball for the initial bounce and returned to physics when it enters the goal square.
Exactly the same process applies though and this will always work - give you a completely 'free-from-constraint'  ball with which to do as you wish.



The video (above) is what happened when kicked with the physics in play and for whatever reason would not behave as needed.

The second (below) when employing a duplicate to bypass the physics altogether..



The procedure -

When kicked, at the frame the foot comes into contact with the ball, stop the animation and duplicate the ball.

Immediately remove the copy's constraints, physics, animation, parentage, everything, so you end up with a free and unconstrained mesh.

Keyframe the copy's location and rotation.

In the Outliner, keyframe the original ball to invisible and the copy to visible.

Go back to frame 1 and keyframe the copy's loc/rot again.   

Also reverse the visual switching in the outliner and keyframe that. (the original is to be seen at frame 1, the copy to be invisible)

Animate the copy and send it into the goal square. (emulate a bounce or two?)

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Whilst unnecessary, for demo purposes here, the original (physics) ball is switched back as the copy crosses the goal threshold.   

Similarly in your project, you can also switch back to the child-ball at anytime.

Switching back - At the frame required...

Move the player-character so the two balls are at the same location. (or close)  Use SH-S to set the 3D cursor to the selected duplicate, then select the original ball  and send it  to the 3D cursor's location. (SH-S again)

This will place both at precisely the same location.  From there it's a simple matter to rotate the child-ball in all 3 axis' until they're aligned again.  I've found it easier to use wireframe mode and "Normal" orientation for this.  Always be in ORTHO view.

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When the balls are textured you have to ensure this is matched at the switch-back frame also -

An easy way to do this is -

Temporarily remove the original ball's smoothing and texture.  Reassign one face to a bright colour before duplicating.  At the switch-back frame rotate both balls so their mis-coloured faces match up. 

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Switching can take place between the two balls as often as needed, all without fear of some constraint's apron strings mucking you around.

The monkey video below has been freely switched back and forth between physics and animation several times, plus throwing in a path constraint for good measure, all without a single glitch.  It's a basic workaround but it always works.

Monkey Blend file is available if you want that.